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Model experiments on macroscopic thermoelectromagnetic convection

Zhang, X.; Cramer, A.; Lange, A.; Gerbeth, G.
The interaction between a thermoelectric current and an imposed magnetic field may produce thermoelectromagnetic convection (TEMC). In the present paper, an experimental study on TEMC in a generic configuration is reported. While the necessary temperature gradient grad T in a square box was accomplished by heating and cooling of two opposing side walls, respectively, utilising a massive nickel plate for the bottom of the electrically conducting container established a material discontinuity with respect to the liquid metal layer. In first place, such a jump in the related Seebeck coefficient non-parallel to grad T is a pre-requisite for the existence of a thermoelectric current. The second condition for TEMC, which is a non-vanishing curl of the Lorentz force, was fulfilled with a permanent magnet producing an inhomogeneous magnetic field. Ultrasonic Doppler velocimetry was used to quantify the TEMC flow field. The measurements demonstrate that even a moderate temperature difference can produce distinct convection. Locating the magnet, the direction of magnetization of which was parallel to grad T, close to either side wall produced a single vortex spreading throughout the entire box. Moving the magnet to the center led to a modified distribution of the magnetic field, which, in turn, altered the flow structure. A convective pattern consisting of four vortices developed and the velocity fluctuations were intensified. The numerical results for the distribution of the magnetic field in presence of the ferromagnetic bottom support the experimental findings.